The present invention is applicable to a type of downhole tool that will be referred to herein as a "stabilizer-type" tool. By this is meant an elongate tool having one or more elongate blades running generally lengthwise along its exterior and projecting radially outwardly, usually to the approximate diameter of the borehole in which the tool is to be used. More particularly, the invention pertains to such a tool of the non-rotary type: the blades do not rotate with respect to the tool body. Such a tool may be a stabilizer per se, or it may, for example, be an MWD tool, such as a density tool in which a blade is provided in register with instrumentation that emits and/or receives radiation or other signals running generally radially between the tool and the formation. Certain aspects of the present invention are applicable to such MWD tools, whether or not they are of the stabilizer type, i.e., whether or not they include radially projecting blade(s).
In a non-rotary stabilizer-type tool, the radially outer surfaces of the blades rub against the formation in use. Indeed, it is ordinarily intended that they do so, for one reason or another. In a stabilizer, such contact may be desired for the purpose of centralizing the adjacent portion of the drill string in the hole. A stabilizer, with its hole-contacting blades, may be used as a pivot point for effecting curvature of a borehole in directional drilling or as a stabilizing element to hold an angle. In an MWD tool, such as a density tool, the hole-contacting blades help to prevent substantial variations in the radial distance between the instruments and the borehole wall, and/or to minimize the thickness of any layer of drilling mud or the like that may be interposed between the tool and the formation and that could adversely affect the precision of the measurements taken by the tool.
Even though the outer surfaces of such blades are typically formed of, or reinforced with, a highly wear-resistant material such as tungsten carbide, they still wear in use. After sufficient wear, they no longer properly perform the functions for which they are intended. If, in a stabilizer, the blades are integrally adjoined to the tool body, then a worn stabilizer must be replaced altogether, or else the blades must be re-dressed.
Either alternative is inconvenient, especially in relatively remote or primitive locations. The size and weight of an entire new tool makes it inconvenient to keep significant numbers of replacement tools on hand, so they must be shipped, which is time-consuming and expensive. Blades cannot be readily redressed in the field, so even if the worn tool is to be refurbished, it must be sent to a plant and a replacement shipped out.
For this reason, there have been a number of past attempts to construct stabilizers with removable, and thus replaceable, blades, which allow a basic tool body to be repaired in the field many times. U.S. Pat. Nos. 3,680,647 to Dixon et al., 3,818,999 to Garrett, 4,106,823 to Bassinger, and 4,378,852 to Garrett disclose various schemes for fitting and attaching individual removable blades to stabilizer bodies. Canadian Patent No. 1,177,057 discloses an interesting variation in which each blade is formed in several segments arranged end-to-end, and in which the underside of the blade may be slotted to provide some squeeze-like flexing deep in the pocket in which the blade is received to ensure a tight fit.
A common problem with such removable blade stabilizers is that fretting and the like can cause failure of the screws or other attachment means that fix the blades to the stabilizer body. Roller reamers have elongate, radially projecting rollers that rotate with respect to the tool body and that may also have lateral and/or longitudinal play in their bushings. Roller reamers may serve stabilizer-like functions. However, with respect to non-rotary stabilizers, the conventional wisdom of the art, exemplified by all of the prior patents cited just above, has been to try to cause the blades to fit as snugly as possible within their pockets, and become rigid with the stabilizer body, in order to resist movement relative to the main body of the tool and ingress of drilling fluid into any gaps between the blade and body. However, problems have persisted.
Where an MWD tool is provided with a stabilizer-type blade overlying its instrumentation, it is, of course, all the more desirable for the blade to be removable. This provides access to the instrumentation. Also, blade wear affects calibration of the tool, and with replaceable blades, expensive instruments, and the specialized tool body that go with them, do not have to be disassembled simply because the outer portion of a blade has become worn.
However, it is equally true that the types of problems described above, e.g. loosening and/or loss of blades, can be even more catastrophic in these very expensive MWD tools.
Some blade designs would take up too much of the available radial space in small diameter tools.
If prior art replaceable-blade stabilizer schemes are applied to such tools, then when a worn blade is re-dressed, substantial warpage may occur, requiring substantial corrective machining to return the tool to proper calibration for its instruments.
Similar problems can occur on elongate hatch doors that may be provided for access to the cavities in the tool bodies in which the instruments are disposed even if they do not serve as blades.
U.S. Pat. No. 4,879,463 to Wraight et al. and European Patent Publication No. 0505261A2 disclose such MWD tools with individual blades.
Another scheme is exemplified by U.S. Pat. Nos. 5,134,285 to Perry et al. and 5,120,963 to Robinson et al., in which all three blades are carried on an annular collar that is threaded onto, and concentrically surrounds, the main tool body. Additional problems that may be presented by this last-mentioned scheme include: rotational and/or axial displacement of the collar with respect to the main tool body, which can interfere with proper registering of the blades with the instrumentation; "over-torquing," which can likewise result in misalignment; size and weight of replaceable part; and complications in re-dressing/re-calibration.
Still another problem in these MWD tools is that of protecting the expensive and relatively delicate instrumentation from harmful forces encountered by the tool in which it is carried, and this can be particularly problematic if the instrument is elongated parallel to the length of the tool itself, since that tool will inevitably bend, or at least experience bending forces, in use. Overriding all of this is the need to provide a pressure seal about the instrumentation.